As I mentioned a couple times before Speer makes a self-defense bullet intended for lower velocity loadings. They call it “Gold Dot® Short Barrel®”. I needed these for handgun students with difficulty handling factory loads. I loaded 301 rounds (I purchased three 100 round boxes and ended up with 301 bullets) over 3.9 grains of Bullseye and delivered 100 rounds to one of my students last Saturday.

Here is what the 180 grain bullets look like in .40 S&W:

I’m expecting a velocity of about 850 fps at the muzzle with my STI DVC (5 inch barrel). This compares to about 1025 fps with 180 grain Winchester Rangers out of the same gun. The difference in recoil is significant.

I’ll run them over a chronograph and do some water jug testing (only valid for simple expansion testing) the first chance I get.

As I reported last weekend I put all my polymer coated lead bullets through a max case gauge and still had problems with my new STI. I had saved one of those cartridges to diagnose the problem.

The cartridge still failed to chamber when stripped off the magazine by the slide when I tried it again at home. The bullet in that cartridge was seated too deep by about 0.040”. None of the others were too deep so I suspect it happened during the chambering of the cartridge. I pulled the bullet and reseated it at the proper depth and it chambered just fine. Hmmm… maybe the crimp just isn’t tight enough, the bullet gets driven deeper into the case, then the cartridge fails to nose down into the chamber as it comes off the magazine.

I shot in another steel match yesterday to get more samples.* On the first stage the first few strings went fine then a round failed to chamber the entire depth. I couldn’t pull the slide back. The gun was essentially locked up. I dropped the magazine, held tight to the slide then pounded the grip forward with the web and palm of my hand. The extractor pulled the round out and it was ejected. I put in a fresh magazine and completed the string in 17.xx seconds. I switched to my Montana Gold JHP handloads for the rest of the match and had no more problems.

I brought home the problem round and put in the case gauge. It fit just fine. I put it in a magazine and tried to to chamber it. It failed to chamber all the way. I tried dropping it directly into the chamber and dropping the slide. It chambered but again I couldn’t extract it without slamming my hand into the grip.

But the round fits just fine in the case gauge!

I measured the round. At the largest point it is 0.425”. The specification for .40 S&W is 0.423. So, it is oversized by 0.002”. But CASE GAUGE!

Hmmm… Maybe I have another case gauge around here… I did. I had lost one for a while and purchased another. I pulled out the other case gauge and the cartridge failed, big time.

These pictures are of the same cartridge in two different case gauges:

In the picture on the left I applied a couple ounces of pressure to get it to seat all the way. In the second picture I put the maximum amount of pressure I could comfortably apply with my thumb to get it in that far.

So why is just the ammo with the polymer coated lead bullets giving me problems?

I measured a few bullets. Depending upon which axis I measure the bullets they have a diameter of 0.400” to 0.403”. The specification is 0.401”. The Montana Gold JHPs I measured have a diameter of 0.399.

Here is the cartridge:

You can’t really see it but you can feel a bulge where the base of the bullet is in the cartridge.

Five conclusions:

1. With a slightly over spec bullet and probably max thickness brass** I end up with an oversized cartridge.
2. The Midway case gauge, on the left, is slightly oversized.
3. My gun has a minimum sized chamber.
4. Two thousands of an inch can make a huge difference in the reliability of a gun.
5. I must use the L.E. Wilson case gauge (on the right above) for this gun.

I might be able to use the Midway gauge for some other gun(s).

Scary thought… Can you imagine needing your gun in a life or death situation and losing the fight because of two thousands of an inch?

* I can’t use this ammo at indoor ranges and I don’t have easy access to any outdoor ranges except when I shoot at matches.

** Not all cartridges with polymer coated lead bullets fail the tighter case gauge. Only some of them fail.

I thought mine was getting complicated and expensive. Actually it’s complete ammunition manufacturing.

The guy giving us the tour obviously isn’t a hand loader, is he?

So anyway; you want a complete home loading facility, it would look something like that. The QA alone is quite an impressive operation.

I’d need a rather larger spare bedroom than the one I currently use for reloading.

Hat tip; Sipsey

The January/February 2015 issue of Front Sight magazine has an interesting article on the statistics of making “power factor*”.

While I certainly had the background in statistics it never occurred to me to apply them to the chronograph data from my hand loaded ammunition to determine the chances of me failing to “make major” at a match. This is despite very nearly failing to make major in the 1998 Area One USPSA match.

Looking at my log files for the ammunition I made for that match I found the following data:

Mean velocity: 992.6 fps
Standard Deviation: 11.3 fps
Bullet Mass: 180 grains
Power Factor: 178.67

Back then you had to have a power factor of 175 to make major and for some reason I thought I had plenty of margin.

At the 1998 Area One match staff pulled eight cartridges at random from the magazines on my belt and tested them as per USPSA regulations. They pulled the bullet from a cartridge and weighed it. They fired three rounds and found I failed to make major. They, as per procedure, fired another three rounds, used the highest three velocities from the six rounds fired and found I was closer but still failed. They had one round left and, as per procedure, asked me what to do with it, “Fire it or weight the bullet?” I had them fire it and using the highest three velocities from the seven rounds fired I just barely made major power factor.

It wasn’t until I read the title of the article in Front Sight article, “The Power of Statistics How to Meet Power Factor with Confidence” that I felt stupid for my experience at Area One.

The bottom line is that your chance of failing the test procedure depends on how many standard deviations you are away from the velocity threshold for the power factor you want to meet.

Using my example from the Area One match the velocity threshold is 972.22 fps (175,000 / 180). My mean velocity was 992.6 fps or 20.378 fps above the threshold. With a standard deviation of 11.3 fps the ammo was 20.378 / 11.3 or 1.8 standard deviations (commonly called ‘Z’) from the threshold. Using a normal distribution table or the article you will discover my chances of failing were about 13%.

Update:

Using this table from the article you will discover my chances of failing were about 13%:

This table is not a standard distribution table. It is a mapping from Z (number of standard deviations away from the mean) to the chances of failing the PF test under USPSA rules. This was obtained using a t-distribution because of the small sample size used by the USPSA regulations. It is assumed the shooter obtained the mean velocity and standard deviation with a sample size of eight.

End update.

I’m going to range today to measure the velocities of a new load I plan to use for competition. I’m going to make sure I’m about 2.5 standard deviations away from the threshold which would put my odds of failing to make major at about 5%.

* Power Factor is defined as the mass of the bullet in grains multiplied by the velocity in feet per second divided by 1000. Or:

Power Factor = bullet weight (grains) x average velocity (feet per second) / 1000

In many competitions your targets are scored differently depending on the power factor of the ammunition you are shooting. For example if you are shooting Limited Class USPSA you “make major” with a power factor of 165 or greater and “make minor with a power factor of 125. For major power factor ‘B’ and ‘C’ zones hit are scored as 4 points and ‘D’ zone hits are scored as two points. If you “make minor ‘B’ and ‘C’ zones hit are scored as three points and ‘D’ zone hits are scored as one point. If you don’t have ammunition which gives you a power factor of 125 or greater all zones are scored as zero. I.E. you aren’t participating in the competition.